Are you shipping from the USA? If not, rather than the incorrect USPS shipping services you are using, scroll down to the International Services section and choose a flat rate that covers the cost of shipping from Canada by Canada Post.
When you are setting up your international shipping, do NOT put USA in with the overseas shipping. It should be its own section. Then you choose Custom Destination again and choose the other countries you will ship too.
Do NOT indicate that you will ship to North and South America. This will override your USA pricing and overcharge your American customers.
Take your time.
When you think you have done a listing correctly, immediately go to it and check it out. (I use United Kingdom and USA= 90210 as my checks since they are close together on the dropdown menu). If there are problems, REVISE. Keep revising until what you want to show is working correctly. The listing will not "index" for several hours, so you are not really losing anything.
EBay has a steep learning curve, and shipping is even steeper. Take your time and don't panic.
